In general, an absorption method, an adsorption method, a membrane separation method, a cryogenic cooling method and the like are used to separate carbon dioxide (CO2) from the exhaust gas of chemical plants, power plants or large-sized boilers and from natural gas. In particular, an absorption or adsorption method is widely used when the concentration of exhausted carbon dioxide is low.
The absorption or adsorption method is widely used since it can selectively separate only some gases that are well absorbed or adsorbed by an absorbent or adsorbent; however, there is a disadvantage that, since the absorbent or adsorbent is chemically altered during the separation process, it is necessary to periodically replace the absorbent or adsorbent. Therefore, in the case of using a solid adsorbent, the chemical alteration of the adsorbent is reduced, and thus it is advantageous for the adsorbent to be applied only when the adsorbent replacement cycle is long. On the other hand, since the absorption method uses a liquid absorbent and thus the absorbent is easy to replace and has a greater absorption capacity than that of the adsorbent, it is widely used in purification of a large amount of exhaust gas or used in gas separation; however, there is still a disadvantage that the liquid absorbent is chemically or thermally altered.
As carbon dioxide absorbents, aqueous solutions containing amines such as monoethanolamine (MEA), diethanolamine (DEA), piperazine or the like are widely used in industry. This is because these amine-based absorbents react with carbon dioxide to easily form stable carbamate compounds, and the carbamate compounds can be decomposed again into carbon dioxide and amine when heat is applied. However, the process for capturing carbon dioxide using these amine-based absorbents has several serious problems. In particular, due to high thermal and chemical stability of carbamate formed by reaction with carbon dioxide, the decomposition temperature is as high as 120° C. or more, thereby causing excessive regeneration energy consumption (MEA requires 4.0 to 4.2 GJ per ton of carbon dioxide), excessive volatile loss of amine due to the high regeneration temperature (4 kg per ton in the case of using MEA), and replenishment of an absorbent.
In order to resolve the drawbacks of the amine-based aqueous solution absorbents, there have been reported various methods of physically absorbing carbon dioxide using organic solvents such as Selexol, IFPexol, NFM, etc. One important advantage of the organic solvent absorbent is that much lower energy is required to recover carbon dioxide and recycle solvents since the absorption of carbon dioxide is achieved only by a physical interaction between the absorption solvent and carbon dioxide, not by the chemical bond as in the case of the aqueous amine absorbents. Actually, in the case of using the amine absorbent, the recovery of carbon dioxide and the recycling of the absorbent require an energy-intensive, high-temperature separating process; however, in the case of the physical absorption, it is possible to recover carbon dioxide dissolved in the solvent by simply changing the pressure, without increasing the temperature. However, the physical absorption process has the following drawbacks.
First, low carbon dioxide absorption capacity: Organic solvents generally exhibit a carbon dioxide absorption capacity at normal pressure that is significantly lower than that of an aqueous amine solution, such that the circulation rate of the absorbent is high, thus requiring relatively large equipment. Therefore, the organic solvent absorbent is more suitable for natural gas purification in which the pressure of carbon dioxide is high.
Second, high circulation rate: Physical absorption process by organic solvents typically requires twice higher absorbent circulation rate compared to amine solutions, thereby requiring more capital and equipment costs.
Therefore, there is a need for the development of a novel absorbent that has high thermal and chemical stability, and has low vapor pressure, so as to overcome the drawbacks of the amine absorbent and the organic solvent absorbent.
Recently, as a method for overcoming the drawbacks of conventional absorbents, attempts have been made to utilize, as an absorbent, an ionic liquid that is non-volatile, has high thermal stability and maintains a liquid phase at a low temperature of 100° C. or less, as disclosed in U.S. Pat. No. 6,849,774, U.S. Pat. No. 6,623,659 and US Patent Application Publication No. 2008/0146849. However, in order to synthesize these ionic liquids, not only complicated manufacturing steps of two or more steps are required but also the manufacturing cost is too high, so that there are many problems in industrial application. In addition, the physical absorbents such as organic solvents and ionic liquids are not suitable for capturing carbon dioxide from the exhaust gas discharged at atmospheric pressure after combustion because of their low ability to absorb carbon dioxide at low pressure.
Therefore, in order to capture carbon dioxide from the exhaust gas after combustion, a chemical absorbent must be always used. However, as mentioned above, the alkanolamine-based chemical absorbent such as MEA has several drawbacks; in particular, there is a problem that excessive regeneration energy is consumed. Recently, attempts have been made to reduce regeneration energy of chemical absorbents, including a method of using, as an absorbent, alkanolamine sterically hindered around amine groups, and a typical example thereof is 2-amino-2-methyl-1-propanol (AMP) which is a primary amine. When reacting with carbon dioxide, AMP forms bicarbonate compounds ([AMPH][HCO3]) that may be regenerated more readily than carbamates, thereby requiring 30% less regeneration energy compared to MEA; however, its CO2 absorption rate is less than 50% of the absorption rate of MEA.
As a method of increasing the absorption rate of AMP, Mitsubishi Heavy Industries, Ltd. and Kansai Electric Power Co., Inc. made a joint effort to develop a novel absorbent prepared by adding piperazine, which is a secondary cycloamine, to AMP (Japanese Patent No. 3197173). However, the absorbent disclosed in this patent has a problem in that precipitation occurs in the process of absorption of carbon dioxide, and when piperazine is reacted with carbon dioxide, thermally stable carbamate compounds are formed in addition to bicarbonates, such that a regeneration process is difficult to perform.
Further, there is also known a method of using, as a carbon dioxide absorbent, alkali carbonate, such as sodium carbonate or potassium carbonate, instead of using a primary alkanolamine absorbent; however, the method has a problem of low carbon dioxide absorption rate. As a method of increasing a carbon dioxide absorption rate, WO 2004/089512 A1 discloses that when piperazine or its derivative is added to potassium carbonate, a carbon dioxide absorption rate is significantly increased; however, this method has a problem such as the formation of precipitate in the process of using potassium carbonate.